Populations of synchronized cells will be prepared by selective detachment of mitotic cells. We will be using for these studies human-mouse somatic hybrid cells and the parental mouse and human cell lines. Quantitation of the amount of enzyme protein present in these cells at different intervals of the cell cycle will be performed for a number of enzymes coded by autosomal or X-linked genes. To measure de novo synthesis, some of these enzymes will also be quantitated by immunoprecipitation. Transcription of human and mouse repeated and unique sequence RNA's during the cell cycle of human-mouse hybrid cells will be analyzed by DNA-RNA hybridization.